Whole blood is refrigerated for storage to between 4.degree. C. to 10.degree. C. for its storage life which ranges between 1-2 months. Other blood products such as plasma, including fresh frozen plasma, and serum may also be refrigerated for storage. The finite storage life of blood precludes stockpiling large amounts of blood and limits the stored supply to blood collected within two months of their use. To further complicate the storage problem, refrigerated blood and blood products must be warmed to body temperature before administration to patients, and once blood has been warmed it cannot be chilled and stored again. In order to avoid wasting blood, various devices have been devised so that blood and blood products can be warmed during transfusion. For the sake of simplicity, the terms blood and blood warmer have been used in describing the invention and liquid warmed therein; however, it is to be understood that the present invention may be used to warm or cool other transfusable liquids, e.g., fresh-frozen plasma, plasma, serum, peritoneal fluid, or any other liquid for which it is desirable to bring the liquid to a specified temperature before infusion to a patient.
Many transfusions are of the gravity type which are performed by attaching a tube continuous with a transfusion bottle or bag suspended approximately 1 M above the patient to a vena puncture site on the patient. The elevated position of the transfusion container relative to the patient provides gravitational pressure on the blood. The flow rate of a gravity transfusion is controlled by applying a screw clamp to the plastic tubing. Depending on the patient's needs, typical gravity transfusion flow rates vary in range from 3 ml/minute. to 30 ml/minute. Gravity transfusion flow rates can be varied rapidly within these ranges by adjusting the screw clamp in response to a change in the patient's medical condition.
Other transfusion methods are used when a patient requires significantly higher transfusion rates. In some cases, manual compression of bolus chambers is used to force a small quantity of blood into a patient at high flow rates. Some surgical procedures, such as thoracic surgery, require bolus transfusions to be performed periodically during surgery. When higher flow rates are required, mechanical pumps may be used to create transfusion flow rates as high as 1 liter per minute. Similarly, heart/lung machines, used during prolonged cardiac surgery, pump, oxygenate, and warm or cool blood while returning it to the patient at high flow rates. In all of these high flow rate transfusion methods, liquid flow rate is often varied quickly in response to the patient's changing medical condition.
It is therapeutically advantageous for traumatized patients to receive blood as close to 37.degree. C. as possible because the significantly cooler or warmer transfusions can cause the patient additional stress. Transfusion with under-warmed bloods may result in hypothermia. Such transfusions can cause a varying degree of hypothermia depending on the volume of blood transfused and its temperature. General anesthetic also causes some degree of hypothermia as a normal side effect. While it is desirable to avoid hypothermia in severely traumatized patients, hypothermia is particularly dangerous if the traumatized patient's body temperature decreases below 35.degree. C. because of the increased risk of ventricular fibrillation. The risk of ventricular fibrillation is heightened during major surgery because both general anesthetic and transfusions are usually required and both can contribute to hypothermia.
While hypothermia is to be generally avoided in traumatized patients, the condition is advantageous in some situations, e.g., to combat a febrile condition or to reduce blood flow and respiration during cardiac surgery. During cardiac surgeries, hypothermia is commonly induced by circulating the patient's blood through a cooling apparatus so that the patient's heart beat is slowed.
Hypothermia is not the only problem in the transfusion warming art, over-warmed blood can be equally harmful to transfusion patients as under-warmed blood. Two possible problems occur with overheating blood, i.e., hemolysis of red blood cells or blood coagulation. Hemolysis can occur when blood temperature attains 42.degree. C. for a significant period of time. Hemolysis can result in poor oxygen carrying capacity and renal difficulties in the patient. Coagulation can be even more dangerous. Blood coagulation may occur when blood temperatures significantly exceeds 42.degree. C. for a significant period of time. Coagulation can result in fluid path occlusions in the warming device or transfusion tubing, or if coagulated blood particles enter the patient, thrombosis may occur. Thus, it can readily seen that the ideal transfusion warmer is one which can maintain a constant blood discharge temperature of 37.degree. C.
Unfortunately, no currently available transfusion warmers can cheaply and efficiently maintain relatively constant discharge temperatures during periods of changing blood flow rates. Procedures such as thoracic surgery lucidly illustrate the problems in the transfusion warming art. During thoracic surgery, small boluses of blood are transfused to the patient by manual compression of a bolus chamber which contains between 10-25 ml of blood so that approximately 10 ml of blood is transfused to the patient at very high liquid flow rates. When serial boluses are required, currently available warming device cannot rapidly and efficiently increase the amount of heat transferred to the blood per unit time it flows through the warmer so that later boluses are discharged at a nearly constant temperature. Thus, in most of the currently available devices increasingly under-warmed blood may be discharged to the patient after successive boluses. After boluses are no longer required, the transfusion rate is drastically decreased and most warming devices overwarm the blood remaining in their liquid chamber because their active heating elements continue to radiate and conduct heat after the heating element is deactivated in response to decreased liquid flow rate. The amount of time required for the device to return discharge temperature to the 37.degree. C. after a change in liquid flow rate is defined as the device's thermal response time. It can be readily seen that the shorter the thermal response time, the less under-warmed or over-warmed blood will be discharged to the patient.
The problems of rapid response time and thermal overshoot were recognized by U.S. Pat. No. 4,707,587 issued to Greenblatt. The Greenblatt apparatus attempts to solve the response time and thermal overshoot problems by using air as the medium of heat conduction. However, the Greenblatt apparatus is both complicated and expensive for dealing with these problems. Unfortunately, using gas as a thermal conduction medium requires the device to include complicated and relatively expensive mechanical devices for operation such as a blood input pressurizer, a gaseous heat exchanger, and electric fans.
U.S. Pat. No. 3,293,868 issued to Gonzalez describes a device designed primarily for cooling peritoneal fluid infusions using a Peltier effect thermocouple as a cooling source (see col. 11 54-59 and col. 11 9-22). The device may alternately be used for warming such fluids by manually activating a switch which reverses the current in the thermocouples. The device is designed so that thermocouples are maintained at constant temperature surrounding thermoplastic conduit through which the liquid flows. Heat is reversibly exchanged from the thermocouple to the fluid contained within a thermoplastic tubing.
Another approach to warming blood is illustrated U.S Pat. No. 4,782,212 issued to Bakke which also exemplifies the prior art's failure to adequately deal with changing blood flow rates. Bakke describes an electrical resistance warmer composed of a thin metal flattened conduit sandwiched between thick metal buffer blocks surrounded by resistance heaters. The current in the resistance heaters is cycled in an attempt to maintain a relatively constant temperature of 37.degree. C. in the buffer blocks. The thickness of the metal buffer blocks prevents rapid response to changing blood flow rates because of long thermal path between the heat source and fluid.
Other problems in the transfusion art are presented by the container which holds the blood while heat is transferred to it. Existing transfusion warmers require air traps downstream from the blood warmer so that air bubbles will not create emboli in the patient. Because air traps require technical training and manual dexterity to properly install, it would be advantageous to make them unnecessary. Because increased liquid flow distance within the container allows a blood warmer to decrease its heating element temperatures, and the prior art taught that lower element temperatures lessen the chance of a dangerous over-temperature conditions, most prior art transfusion containers had labyrinth internal structures. Labyrinth internal structures are more likely to trap air bubbles in their complicated structure and cause substantial turbulence in the liquid flow. Turbulence should be avoided in blood infusions because turbulence can cause damage or lysis of red blood cells.
Additional problems in the transfusion art are created by mobile transfusions required in medical emergencies. During such emergencies, transfusions are often begun at the scene of the injury or during transportation to medical facilities. Military corpsman, paramedics, and ambulance attendants often provide emergency transfusions in ambulances or medical evacuation helicopters. For these mobile transfusions a small portable transfusion warmer is desirable.
Further, most prior art warmers require relatively large amounts of A.C. power which make them impractical for mobile use since sufficient A.C. power is not typically available in the field or on board the medical transportation.
An additional problem in the transfusion art is created by transferring patients between areas of a hospital while transfusing chilled blood. In many emergency room situations, a patient must be transferred to the operating theatre quickly while continuing an infusion of chilled blood. If a warmer is using A.C. power, the power cord must be extended to move the patient while continuing blood warming or the cord may be unplugged. Thus, it would be advantageous if the blood warmer power supply could be switched from A.C. to D.C. for a short period so that the patient could be moved without interrupting the blood warming, and there would be no need for a long power cord.